Additive manufacturing, colloquially called “3-D printing,” is an emerging technology that has the potential to completely transform modern manufacturing processes. In the most common conventional manufacturing processes, an object is manufactured by cutting/shearing components of the object from a material, followed by a shaping process that may include bending or molding. These processes are labor intensive and costly. Additive manufacturing bypasses these drawbacks by printing the object directly to its final form. The printing technology is fairly straightforward. An industrial robot is controlled by a computer to print a three-dimensional object from a computer model by depositing and curing plastics, metals, ceramics, or a combinational thereof layer-by-layer on a flat surface.
There are numerous benefits to additive manufacturing over conventional manufacturing processes. First, because additive manufacturing is based on computerized models, there is virtually no limit to the types and complexity of objects that can be printed, so long as the object does not exceed the physical limitations of the materials and the printer, such as the dimensions of the printing surface. Second, complex objects can be printed in one job as one object, eliminating typical labor-intensive and high cost post-fabrication assembly processes. Third, additive manufacturing does not require any production of specialized tools, which are a significant cost associated with conventional manufacturing processes. Fourth, additive manufacturing offers customization at almost no additional cost, since end-user personalization can be met simply by modifying the computerized model of the object prior to printing. Fifth, additive manufacturing creates less waste, since objects are manufactured in essentially final form and with high precision, avoiding the cutting and reshaping that are a necessary feature of conventional manufacturing processes. Sixth, additive manufacturing allows for the creation of multi-density or multi-material objects that would be impossible through any other manufacturing means.
Fused filament fabrication (FFF) is one type of additive manufacturing that is gaining rapid popularity in prototyping, modeling, and production applications. In FFF operation, a polymer filament is pulled from a storage mechanism, often times a cylindrical spool, into a printing head, called an extrusion nozzle. A toothed gear drive pushes the filament at a controlled rate into the nozzle head, which is heated. As the filament moves through the nozzle head it melts and is extruded from the nozzle head in the form of beads. As the beads pass the nozzle head they contact ambient air and immediately cool and harden as they are being deposited onto the print surface. The nozzle head is controlled by a computer to move along the X-Y axes, while the table surface is controlled to move along the Z-axis. The nozzle and table follow a routine that is based on a computer design of an object so as to print the object from bottom-to-top, layer-by-layer.
The single most critical component in FFF additive manufacturing is the nozzle system. The nozzle system can effect and dictate printing speed, accuracy, and precision. FIGS. 1A-1B illustrate two representative prior art nozzle systems 100, 101. FIG. 1A depicts an example of a Teflon-lined J-Head nozzle system 100, which includes a cold end called the heat sink 120, and a hot end called the nozzle head 160. An inner bore 138 traverses the heat sink 120 and nozzle head 160, and is sized to receive a polymer filament 110. The purpose of the heat sink 120 is to maintain the polymer filament 110 at room temperature and thus in its most solid form up until the point it makes contact with the nozzle head 160. The heat sink 120 is separated from the nozzle head 160 by what's called a “thermal break” 150. The purpose of the thermal break 150 is to distance the heat sink 120 from the nozzle head 160 so that the heat sink 120 is shielded from heat that migrates from the nozzle head 160 during printing. In the J-Head nozzle system 100, the inner bore 138 is coated with TEFLON® to ensure that the filament 110 passes through the bore 138 smoothly, and to ensure that heat is not inadvertently passed on to the filament 110 as it is fed through the heat sink 120. The nozzle head 160 includes a heater block 192 and is often made of aluminum or brass. The nozzle head 160 also includes a temperature changing resistor (thermistor) (see thermistor mounting location 193) for taking temperature measurements. FIG. 1B depicts an example of an all-metal nozzle system 101. This system similarly includes a cold heat sink end and a hot nozzle head end separated by a thermal break 151. In this nozzle system 101, however, the inner chamber is made of steel and the nozzle head 161 is made of brass. No TEFLON® exists in this variant of design.
Current FFF nozzles, such as those described above, suffer from numerous drawbacks. Most nozzle systems on the market are made from either aluminum or brass, which are low cost materials that very efficiently conduct heat. However, in FFF, heat conduction actually works as a detriment to high performance. Specifically, due to the high conductivity of these materials, heat can very easily and quickly migrate from the nozzle head to the heat sink through the thermal break region. The unintended result is that the polymer filament can be warmed in the heat sink before it enters the nozzle head. If the temperature in the filament rises to a critical value, the plastic softens dramatically before it ever reaches the nozzle head. This results in a poor grip of the toothed drive wheel as it drives the filament through the heat sink. This also leads to an uneven flow of material into the nozzle head or, in the worst case, a total system jam. Furthermore, due to heat migration, conventional nozzle systems made of brass or aluminum cannot operate at high temperatures, limiting the types of polymer filament materials that can be used for printing. Finally, brass and aluminum are soft materials with low yield strengths and poor hardness, so they tend to deteriorate quickly as a side effect of the additive manufacturing process. This deterioration process is accelerated by abrasive additives that could be contained within the filament.
Another problem with prior art nozzle systems is their reliance on TEFLON® in the inner bore 138 of the heat sink 120, such as the nozzle system 100 depicted in FIG. 1A. Currently, most nozzle systems use TEFLON® in the inner bore due to its slippery nature and low heat conductance. TEFLON® works well for temperatures below 250° C. At higher temperatures, however, the TEFLON® begins to toxically decompose. This drawback limits the types of polymer filaments that can be used for printing, including high-performance plastics. The prior art nozzle system 101 depicted in FIG. 1B does not use TEFLON® in the inner bore 139 of the heat sink 123. However, while this design is capable of printing higher temperature polymers, it is more prone to heat migration through the thermal break, thus causing a softening of the polymer filament 111 within the heat sink 123 before reaching the nozzle head 161 and ultimately leading to jamming and clogging of the nozzle system 101.
Yet another drawback of conventional nozzle systems is their inability to maintain a consistent temperature in the nozzle head, which ultimately leads to repeatability issues. Most nozzles use a temperature changing resistor (“thermistor”) to measure and track the nozzle head temperature. A thermistor is a glass encapsulated bead that must be mechanically attached on the external face of the nozzle. Because the thermistor is positioned externally, it is subject to ambient air flow, poor contact, and slow transient response, all of which lead to gross errors in temperature readings. Errors in temperature readings make it difficult to maintain the nozzle head at a consistent temperature. If the nozzle head temperature fluctuates, the consistency of the filament may fluctuate causing variance in the quality of deposited layers. Additionally, thermistors are inconsistently mounted from nozzle to nozzle. Because each nozzle head is therefore unique, each machine necessarily requires temperature settings that are tailored to the particular nozzle head used in the machine. This uniqueness introduces an error quantity that has to be accounted for by a human operator, thus reducing the overall system performance and reliability.
In prior art nozzle systems, the nozzle head, heater block, heat sink, and thermal break are coupled via mechanical interfaces such as threads and clamps. These components are often made up of different materials. There are a number of drawbacks in this design. Thermal cycling caused by the additive manufacturing process can eventually loosen the threads causing molten plastic to weep out of the nozzle head, thereby interfering with the polymer deposition and quality of the print job. Furthermore, the varying expansion rates of the materials in these threads and clamps causes gaps to form in these components. Melted polymer filament can become trapped in these gaps, causing jams in the filament feed mechanism and along remaining portions of the filament path.
With the combination of the afore-mentioned prior art drawbacks, prior art nozzle systems tend to have very poor system reliability. In FFF additive manufacturing, however, reliability must be at a minimum of three sigma (99.7%) to achieve an acceptable printing performance. This reliability requirement is necessary in light of the nature of additive manufacturing, where one print job may involve hundreds of hours of printing and the process is zero-fault tolerant. Thus, there is need in the art for nozzle systems for FFF additive manufacturing that can achieve a substantially high and consistent system reliability.
Thus, it is desirable to provide a nozzle system for use in an additive manufacturing system for fabricating an object and method of manufacturing same that are able to overcome the above disadvantages.
Advantages of the present invention will become more fully apparent from the detailed description of the invention hereinbelow.